This is a bit of myth. I would expect a good steel road / touring bike with wider tyres to be more comfortable than a slicked up MTB. Obviously a full on race bike with high pressure 23mm tyres isn't going to like being ridden down a pot holed road. Although, there's no reason why it can't be.
http://images.google.co.uk/images?s...paris roubaix&um=1&ie=UTF-8&sa=N&hl=en&tab=wi
Most tubes will be labelled with a range. Say 26x1.95-2.5, whereas tyres just have the one size (obviously).
If your tubes haven't got a range listed on them (Unusual) it's worth just trying them in a 1.95 tyre anyway. You're only talking 0.5" difference, which isn't much for a tube that stretches to fit.
You can probably just turn your existing stem upside down. So instead of sloping up it is level.
Padding doesn't make a saddle comfortable but a little bit definately helps with the jarring / vibration you get using hardtail off-road. For that reason I wouldn't recommend a full on carbon road saddle off-road; there's no reason why you couldn't get a lighter saddle though.
Good choice. You will notice a huge increase in speed.
BTW, it's tyres not tiers (which means levels). Don't worry, I can't spell either!.